The preparation and use of water-dispersible polyisocyanates, i.e., isocyanates with at least two NCO groups in the molecule, is well-known (see EP-A 540,985 and documents cited therein). They are gaining increasing economic importance for the preparation of lacquers and coating agents based on polyurethane which emit a substantially lower proportion of volatile organic components (VOC), in particular solvents, during application. Due to their low tendency to become yellow under the influence of light, representatives which contain exclusively (cyclo)aliphatically bonded NCO groups are gaining particular importance for the preparation of high quality lacquer raw materials and coating agents.
A variety of substances and methods are available for dispersing in water those polyisocyanates which are hydrophobic, per se, and react with water, these being known from "classical" two-component technology, i.e., those normally requiring substantially non-aqueous conditions.
Thus, it is possible to disperse classical polyisocyanate hardeners, e.g., derivatives of aliphatic diisocyanates such as hexamethylene diisocyanate (HDI), isophorone diisocyanate (IPDI) or hydrogenated 4,4'-diphenylmethane diisocyanate (H.sub.12 MDI, Desmodur.RTM. W=a commercial product from Bayer AG) in water by using extremely high shear forces. The last two compounds mentioned, however, always require the use of solvents since the 100% solids systems are solids or highly viscous oils. These dispersions produce high quality coatings which are substantially equal to those which result from the use of non-aqueous processing technologies (see L. Kahl, M. Bock, E. Jurgens and H. -J. Laas in "Farbe und Lack", 102nd issue, March 1996, pages 88-100 and literature cited therein).
The disadvantage of these is the cost of the nozzle jet dispersion process since these technologies are not available in all fields of application, in particular car repair lacquering.
On the other hand, according to the disclosure in EP-A 540,985, the chemical incorporation of hydrophilic components into the above-mentioned classical polyisocyanate hardeners has been proposed in order to overcome the hydrophobic nature of these species which cannot be directly dispersed under the effect of low shear forces (e.g., stirring in by hand). The incorporation of these hydrophilic components, however, often takes place by using the free NCO groups in the polyisocyanate (e.g., by urethenization with specific, e.g., monofunctional, polyethers). Thus the provision of hydrophilic characteristics is naturally associated with a lowering of the NCO functionality of the polyisocyanate hardener and therefore a drop in the concentration of valuable NCO groups which are available for cross-linking to the substrate.
Furthermore, polyisocyanates have also been rendered hydrophilic by adding so-called "external" emulsifiers (e.g., EP-A 557,844 and documents cited therein). Apart from the economic disadvantage of an additional process step, this method involves the critical disadvantage that the external emulsifiers used have to remain in the lacquer film and tend to affect its physical properties disadvantageously, e.g., due to so-called "sweating," due to the higher degree of hydrophilicity and, resulting from that, a greater sensitivity of the lacquer or coating towards the effects of water, odor problems due to the frequently intense and unpleasant intrinsic odor of the emulsifiers or of the products formed from it, etc.
It is known that the viscosity of (poly)isocyanates is connected with their water dispersibility (e.g., EP-A 0,358,979). That is, low viscosity should generally lead to better dispersibility. However, this relationship is not generally applicable as can be seen from a few comparative trials (1.c. Example 1).
Also, explicit reference is made for example in EP-A 0,358,979 (p.2,line 54 to p.3, line 1) that the NCO-reactive components, and the optional auxiliary substances and additives which are used, have to be present before adding the isocyanate component to the aqueous polymer solution and/or dispersion. Thus, it can be assumed that these auxiliary substances and additives do not have an uncritical effect on the dispersibility of the polyisocyanate which itself is not water dispersible in the absence of additives.
The object of the present invention is to provide water-dispersible polyisocyanates or polyisocyanate mixtures which are characterized by ease of incorporation, even at low shear forces (e.g., "stirring by hand"), and which contain no hydrophilic additives.